Fix a resource leak in uu_avl_pool_destroy

Need to destroy the pthread mutex created in uu_avl_pool_create.

https://svnweb.freebsd.org/base?view=revision&revision=262912

Obtained from: FreeBSD
Sponsored by: Spectra Logic Corporation
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #11528